Fuel cell system and method of stopping the system

ABSTRACT

When a fuel cell system is to be stopped, a controller  3  decreases the coolant flowrate to a fuel cell  1 , decreases the cooling performance of the fuel cell  1  and continues operation of the fuel cell  1 , and performs a temperature rise operation wherein the temperature of the fuel cell  1  is increased using the heat generated by the electrochemical reaction in the fuel cell  1 . After the temperature of the fuel cell  1  has reached a predetermined high temperature, the operation of the fuel cell  1  is stopped.

FIELD OF THE INVENTION

[0001] This invention relates to a fuel cell system, method of stoppingit and to prevention of freezing of the fuel cell.

BACKGROUND OF THE INVENTION

[0002] In some fuel cells such as a polymer electrolyte fuel cell, whichgenerates power using energy produced by a chemical reaction betweenhydrogen and oxygen, it is necessary to humidify the fuel cell to permitthe fuel cell to function. The fuel cell itself generates heat when itgenerates power, so a cooling system is required which cools the fuelcell by cooling water or the like.

[0003] In a fuel cell system comprising a fuel cell using water, if thesystem is left below freezing point when it has stopped, the water orcooling water in the cell freezes and can cause damage or interfere withrestart of the fuel cell system. JP7-169475A published in 1995 andJP11-214025A published by the Japanese Patent Office proposes a methodfor preventing freezing of the fuel cell.

SUMMARY OF THE INVENTION

[0004] However, according to the methods of the prior art, when the fuelcell temperature dropped or the outside air temperature dropped, thefuel cell was heated by a burner or the like from outside or it wasautomatically restarted. If this type of processing is automaticallyperformed during stop regardless of the driver's intention, it is notvery desirable from the viewpoint of safety, etc.

[0005] It is therefore an object of this invention to heat a fuel cellduring system stop and prevent freezing of the fuel cell without theneed to heat or restart.

[0006] In order to achieve above object, the present invention providesa fuel cell system, comprising a fuel cell which generates power by anelectrochemical reaction, a cooling mechanism which cools the fuel cell,and a controller which, when the system is to be stopped, functions todecrease the cooling performance of the cooling mechanism andcontinuously operate the fuel cell to raise the temperature of the fuelcell using the heat of the electrochemical reaction, and stop operationof the fuel cell after the temperature of the fuel cell has risen.

[0007] According to an aspect of the invention, the present inventionprovides a method of stopping a fuel cell system provided with a fuelcell which generates power by an electrochemical reaction and a coolingsystem which cools the fuel cell, comprising decreasing the coolingperformance of the cooling device of the fuel cell and continuouslyoperating the fuel cell before the system stops, and raising thetemperature of the fuel cell using the heat generated by theelectrochemical reaction of the fuel cell.

[0008] The details as well as other features and advantages of thisinvention are set forth in the remainder of the specification and areshown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic view of a vehicle comprising a fuel cellsystem according to this invention.

[0010]FIG. 2 is a schematic view of the fuel cell system.

[0011]FIG. 3 is a flowchart showing the details of a temperature riseoperation of the fuel cell.

[0012]FIG. 4 is a time chart showing the fuel cell temperature variationafter system stop.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Referring to FIG. 1 of the drawings, a vehicle with a fuel cellsystem 1 according to this invention comprises a controller 3 whichcontrols the fuel cell system 1, a battery 2 charged by power generatedby the fuel cell system 1, and switches 5, 6. The fuel cell system 1 isconnected to a load 4.

[0014] The fuel cell system 1 is a polymer electrolyte fuel cell (PEFC)(or phosphoric acid fuel cell (PAFC)), and it is a fuel cell whichrequires humidification during operation. The controller 3 comprisesone, two or more microprocessors, a memory, and an input/output port.When the fuel cell system 1 is to be stopped, the controller 3 performstemperature rise operation of the fuel cell system 1 describedhereafter.

[0015] The load 4 is a motor which receives power from the fuel cellsystem 1 and generates drive force. The drive force of the motor istransmitted to the drive wheels of the vehicle via the vehicle shaft.

[0016] The fuel cell system 1 can be connected to the load 4 and battery2 via the switches 5, 6. When the system is running, the fuel cellsystem 1 and the load 4 are connected, and during temperature riseoperation which is performed when the fuel cell system 1 is to bestopped, the fuel cell system 1 and battery 2 are connected. The battery2 is a chargeable secondary cell such as a nickel cadmium battery,lithium battery or nickel hydrogen battery.

[0017]FIG. 2 shows the schematic construction of the fuel cell system 1.A fuel cell 11 of the fuel cell system 1 has a stack constructionwherein plural cells are laminated. In the fuel cell 11, fuel gas suchas hydrogen is supplied to the cathode, and an oxidizing agent gas suchas air, oxygen is supplied to the anode. The fuel cell 11 obtainselectromotive force by the following chemical reactions:

H₂→2H⁺+2e⁻  (1)

(½)O₂+2H⁺+2e⁻→H₂O  (2)

H₂+(½)O₂→H₂O  (3)

[0018] Equation (1) is an anode reaction, and equation (2) is a cathodereaction. The equation (3) is a chemical reaction which occursthroughout the fuel cell 11.

[0019] The fuel cell system 1 further comprises a pipe 12 which suppliesfuel gas, a pipe 13 which supplies oxidizing agent gas, and a pipe 14which supplies a coolant (cooling water), a heat exchanger 16 whichperforms heat exchange between the coolant and the outside air, acoolant tank 17, a valve 15 which adjusts the flowrate of the coolant, apump 18 which circulates the coolant, a temperature sensor 19 whichdetects the temperature of the fuel cell 11 and a pipe selection valve20.

[0020] When the fuel cell system 1 is to be stopped, the coolingfunction of the fuel cell 11 is reduced, the above electrochemicalreactions are continued, and the temperature of the fuel cell 11 risesdue to the heat generated by the fuel cell 11. A membrane electrode, agas diffusion layer and a separator, which are used in PEFC, have heatresistance up to a certain temperature, so the fuel cell 11 can beheated to the heat resistance temperature.

[0021] The heat exchanger 16 has the function of lowering thetemperature of the coolant which has passed through the fuel cell 11,but when the temperature of the fuel cell 11 is being raised, thecooling function may be reduced by lowering the circulation flowrate ofthe coolant. Further, the coolant may be made not to pass through theheat exchanger 16 by changing the pipe selection valve 20, in which casethe cooling function of fuel cell 11 is further reduced and thetemperature rise effect of the fuel cell 11 is enhanced. Consideringthat steam will accumulate in the fuel cell 11 when the temperaturerises, a gas purge outlet may be installed in the fuel cell 11.

[0022] Next, the temperature rise operation of the fuel cell 11performed when the fuel cell system 1 is to be stopped, will bedescribed referring to the flowchart showing in FIG. 3. This flowchartis executed by the controller 3 when the fuel cell system 1 is to bestopped.

[0023] When the fuel cell system 1 is to be stopped, firstly, a celltemperature (hereafter, fuel cell temperature) Tcell of the fuel cell 11detected by the temperature sensor 19, and an outside air temperatureTout detected by a temperature sensor 22, are read (step S1). A stoptime te predicted by the driver and input via an input device 21connected to the controller 3 is read (step S2), and it is determinedwhether or not temperature rise operation of the fuel cell should beperformed based on this and the outside air temperature Tout.

[0024] Specifically, a time (hereafter, cooling time) tc until the fuelcell 1 cools by naturally losing heat to a predetermined low temperatureTlow (e.g., 0° C.), is estimated (step S3). Thus, by determining whetheror not this cooling time tc is longer than the system stop time te, itis determined whether or not it is necessary to perform a temperaturerise operation (step S4).

[0025] When the system stop time te is shorter than the cooling time tc,the fuel cell temperature Tcell does not fall to the above predeterminedlow temperature Tlow on the next startup even if temperature riseoperation is not performed, so the fuel cell 11 is stopped withoutperforming temperature rise operation (steps S4→S15). In this way, anunnecessary temperature rise operation is prevented, and fuelconsumption is suppressed. However, even in this case, a suitabletemperature rise may be given by referring to the temperature variationhistory of the outside air temperature Tout. For example, if the outsideair temperature Tout is falling and it is determined that the fuel celltemperature Tcell can fall to the predetermined low temperature Tlow,the temperature rise operation may be performed.

[0026] On the other hand, when the stop time te is longer than thecooling time tc, a temperature rise operation is performed, and the fuelcell temperature Tcell is raised to a predetermined high temperature T*so that the temperature of the fuel cell 11 does not fall to thepredetermined low temperature Tlow at which freezing occurs until thesystem restarts (steps S6-S14).

[0027] Here, as the rate at which the fuel cell temperature Tcell falls,increases the lower the outside air temperature Tout becomes or thelonger the system stop time te becomes, the predetermined hightemperature T* is set to a higher temperature, the lower the outside airtemperature Tout becomes and the longer the system stop time te becomes(step S6). Also, the predetermined high temperature T* may be varieddepending on whether or not the outside air temperature Tout falls belowfreezing point.

[0028] If the system stop time te is long, as the stop time te is toolong compared to the cooling time tc, it may occur that the temperaturecannot be raised to the temperature T* required to prevent freezing evenif all the residual fuel amount F1 is used for the temperature riseoperation. In such a case, the temperature rise of the fuel cell 11 istherefore decreased or the temperature rise operation is interrupted. Todecrease the temperature rise of the fuel cell 11, the temperature riseoperation may be shortened or cooling performance losses may be reduced.The reason for this is that temperature rise operation would be uselesseven if temperature rise operation was performed. Specifically, theresidual fuel amount F2 required to raise the fuel cell temperatureTcell to the predetermined temperature T* is computed (step S7), andwhen the required fuel amount F2 is larger than the residual fuel amountF1 read in the step S5, temperature rise running is performed onlyslightly or not at all (steps S11→S15). The required fuel amount F2 canbe computed based on the current fuel cell temperature Tcell.

[0029] When the predetermined high temperature T* is set to such a hightemperature that it affects the fuel cell components (T*>Tuplim), itcauses the functional degeneration of the fuel cell 11, so temperaturerise of the fuel cell due to temperature rise operation is reduced ornot performed at all (steps S8→S15). The upper limiting temperatureTuplim can be previously read into the control program, and this can beset within a range so that it does not affect the deterioration of thecomponents of the fuel cell 11.

[0030] The residual fuel amount F1 read in the step S5 is the residualamount of hydrogen in a direct hydrogen fuel cell system where storedhydrogen is directly supplied to the fuel cell, and is the residualamount of hydrocarbon fuel in a reformate fuel cell system wherehydrogen obtained by reforming a hydrocarbon fuel such as methanol issupplied to the fuel cell. This is detected by a residual fuel amountsensor 23 attached for example to a fuel tank. The residual fuel amountF1 may also be calculated by calculating the fuel consumption amountwhen the tank is full, and subtracting the fuel consumption amount fromthe maximum capacity of the fuel tank.

[0031] When the system stop time te is longer than the cooling time tc,and a temperature rise operation is required within a range permitted bythe fuel amount F1 (F1>F2), the temperature rise operation is performeduntil the temperature of the fuel cell 1 rises to the predeterminedtemperature T* (steps S9-S14). The temperature rise operation isperformed by decreasing the opening of the valve 15 to decrease theflowrate of coolant to the fuel cell 11, or by stopping the pump 18 soas to stop the supply of coolant to the fuel cell 11 and operate thefuel cell 11 continuously by supplying fuel to the fuel cell 11.

[0032] When the predetermined temperature T* is high and it is requiredto enhance the temperature rise effect, the coolant is not passedthrough the heat exchanger 16 by changing the pipe selection valve 20.Further, when T* is higher than the boiling point Tb of the coolant andit is required to raise the fuel cell temperature Tcell above Tb, thecoolant is discharged from the fuel cell 11, the fuel cell 11 isoperated and the temperature of the fuel cell 11 is thereby increased(steps S9→S10).

[0033] This temperature rise operation is continued until the fuel celltemperature Tcell detected by the temperature sensor 19 reaches thepredetermined high temperature T*, and when it reaches the predeterminedtemperature T*, supply of fuel gas and oxidizing agent gas to the fuelcell 11 is stopped and operation of the fuel cell 11 is stopped (stepS15).

[0034] Here, the temperature rise operation was continued until the fuelcell temperature Tcell reaches the predetermined temperature T*, howeverthe time required to heat the fuel cell 11 from a certain temperature tothe predetermined temperature T* can previously be stored in the formof, for example, a table or map in the memory of the controller 3, andit may be determined that the fuel cell temperature Tcell has reachedthe predetermined temperature T* when the elapsed time from temperaturerise operation startup reaches a predetermined time obtained by lookingup the table or map.

[0035] The power generated by the fuel cell 11 in the temperature riseoperation is stored by the battery 2, so it is possible to raise thetemperature of the fuel cell 1 with little energy loss. The power storedby the battery 2 may for example subsequently be used as power torestart the system or power used while the fuel cell 11 stops.

[0036]FIG. 4 shows the results of a fuel cell temperature dropexperiment carried out to investigate the effect of temperature riseoperation according to this invention. In the experiment, a fuel cellcomprising ten cells insulated by an adiabatic material was placed in aconstant temperature room at a fixed temperature of −10° C., andinsulation was also placed around the pipes to avoid heat losses as faras possible.

[0037] The solid line in the figure shows the fuel cell temperaturevariation when this invention was not applied, i.e., the fuel cell wasoperated continuously at a current density of 1A/cm² until the celltemperature and temperature of the circulating coolant were constant(333K), the fuel cell was stopped, and then left as it was. On the otherhand, the broken line in the figure shows the fuel cell temperaturevariation when this invention was applied, i.e., the coolant circulationflowrate was controlled to raise the temperature of the fuel cell, thefuel cell was stopped when the cell temperature reached 368K, and thenleft as it was. In both cases, the start time-when the fuel cell wasleft was taken as zero.

[0038] As shown in FIG. 4, if the fuel cell temperature is increasedwhen the fuel cell is to be stopped, the fuel cell temperature ismaintained high after it has stopped and the time at which the fuel cellbegins to freeze can be delayed as shown by the arrow X in the figure.According to this invention, the fuel cell temperature is sufficientlyincreased before the fuel cell stops depending on the outside airtemperature, so the fuel cell can be restarted before the fuel cellfreezes, and freezing of the fuel cell is prevented.

[0039] It may be noted that the time until freezing temperature isreached can be further extended by applying suitable insulation betweenthe fuel cell and the exterior as in this embodiment.

[0040] The entire contents of Japanese Patent Application P2001-347337(filed Nov. 13, 2001) are incorporated herein by reference.

[0041] Although the invention has been described above by reference to acertain embodiment of the invention, the invention is not limited to theembodiment described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inthe light of the above teachings. The scope of the invention is definedwith reference to the following claims.

INDUSTRIAL FIELD OF APPLICATION

[0042] This invention can be applied to various types of fuel cellincluding those used in vehicles, and is effective for preventingfreezing when the fuel cell system is stopped in an environment belowfreezing point. In particular, a burner does not operate automaticallywhile the system is stopped and the system is not automaticallyrestarted, so the safety of the vehicle when applied to a vehicle fuelcell system is enhanced.

1. A fuel cell system, comprising: a fuel cell (11) which generatespower by an electrochemical reaction, a cooling mechanism (15-18) whichcools the fuel cell (11), and a controller (3) which, when the system isto be stopped, functions to: decrease the cooling performance of thecooling mechanism (15-18) and continuously operate the fuel cell (11) toraise the temperature of the fuel cell (11) using the heat of theelectrochemical reaction, and stop operation of the fuel cell (11) afterthe temperature of the fuel cell (11) has risen.
 2. The fuel cell systemas defined in claim 1, further comprising a sensor (19) which detectsthe temperature of the fuel cell (11), wherein the controller (3)further functions to stop the temperature rise operation when thetemperature of the fuel cell (11) has reached a predeterminedtemperature.
 3. The fuel cell system as defined in claim 1, wherein thecontroller (3) further functions to stop the temperature rise operationwhen it is determined that the temperature of the fuel cell (11) hasreached a predetermined temperature based on the time for which thetemperature rise operation has continued.
 4. The fuel cell system asdefined in claim 2 or 3, further comprising a sensor (22) which detectsthe outside air temperature of the fuel cell (11), wherein thecontroller (3) further functions to set the predetermined temperature tobe higher, the lower the outside air temperature is.
 5. The fuel cellsystem as defined in claim 2 or 3, further comprising an input device(21) for inputting a stop time after the system stops until the systemis next restarted, wherein the controller (3) further functions to setthe predetermined temperature to be higher, the longer the stop time is.6. The fuel cell system as defined in claim 2 or 3, further comprising:an input device (21) for inputting a stop time after the system stopsuntil the system is next restarted, a sensor (19) which detects thetemperature of the fuel cell (11), and a sensor (22) which detects theoutside air temperature of the fuel cell (11), wherein the controller(3) further functions to set the predetermined temperature based on thestop time, fuel cell temperature and outside air temperature.
 7. Thefuel cell system as defined in claim 2 or 3, wherein: the controller (3)further functions not to perform temperature rise operation when thetemperature of the fuel cell (11) cannot be increased to thepredetermined temperature even if all the remaining fuel is used for thetemperature rise operation.
 8. The fuel cell system as defined in anyone of claims 1 to 3, further comprising: an input device (21) forinputting a stop time from when the system stops to when the system nextrestarts, and the controller (3) further functions to: estimate a timefrom when the system stops to when the fuel cell (11) freezes based onthe temperature of the fuel cell (11) and the outside air temperature,and not perform the temperature rise operation when the stop time isshorter than the estimated time to freezing.
 9. The fuel cell system asdefined in claim 2 or 3, wherein the controller (3) further functions todecrease the temperature rise of the fuel cell (11) during temperaturerise operation, when the set predetermined temperature is higher than athreshold temperature determined according to the characteristics of thecomponents of the fuel cell (11).
 10. The fuel cell system as defined inclaim 2 or 3, wherein the controller (3) further functions not toperform temperature rise operation, when the set predeterminedtemperature is higher than a threshold temperature determined accordingto the characteristics of the components of the fuel cell (11).
 11. Thefuel cell system as defined in claim 2 or 3, wherein the coolingmechanism (15-18) cools the fuel cell (11) by supplying a coolant to thefuel cell (11), and the controller (3) further functions to: performtemperature rise operation of the fuel cell (11) after the coolant hasbeen discharged from the fuel cell (11) when the set predeterminedtemperature exceeds the boiling point temperature of the coolant. 12.The fuel cell system as defined in any one of claims 1 to 3, furthercomprising a secondary cell (2) having charging functions, and thecontroller (3) further functions to charge the secondary cell (2) withpower generated by the fuel cell (11) in the temperature rise operation.13. The fuel cell system as defined in any one of claims 1 to 3, whereinthe fuel cell (11) is one of a polymer electrolyte fuel cell and aphosphoric acid fuel cell.
 14. A method of stopping a fuel cell systemprovided with a fuel cell (11) which generates power by anelectrochemical reaction and a cooling system (15-18) which cools thefuel cell (11), comprising: decreasing the cooling performance of thecooling device (15-18) of the fuel cell (11) and continuously operatingthe fuel cell (11) before the system stops, and raising the temperatureof the fuel cell (11) using the heat generated by the electrochemicalreaction of the fuel cell (11).
 15. A method of stopping a fuel cellsystem as defined in claim 14, further comprising: stopping operation ofthe fuel cell (11) after the temperature of the fuel cell (11) hasrisen.